Shallow Geophysical Techniques Research Articles

Shallow geophysical and hydrological investigations to identify groundwater contamination in Wadi Bani Malik dam area Jeddah, Saudi Arabia

Abstract The integration of geophysical techniques with hydrological investigation is frequently used for solving different geological and environmental problems including groundwater quality and exploration and seismic vulnerability assessment. In this research, the shallow geophysical techniques comprising of electrical resistivity profiling, vertical electrical sounding, and ground magnetic were used to identify the contaminated areas lying in the upstream and downstream of Almisk Lake in Jeddah. The chemical analyses of water samples collected from the wells located in the downstream and upstream areas were used to support these results as an increase in the total dissolved solids (TDS) shows a decrease in the resistivity value. The results of geophysical techniques and hydrochemical analyses show that the TDS values are significantly higher in the upstream area of dam than those of the downstream, which suggests that the contaminating source is lying in the upstream of the reservoir. Moreover, the dam was not completely successful to block the contamination because of improper base. The hydrochemical analysis and geophysical results clearly indicate that the groundwater is not suitable for drinking and irrigation purposes.

Subsidence mechanisms and sedimentation in alluvial sinkholes inferred from trenching and ground penetrating radar (GPR). Implications for subsidence and flooding hazard assessment

Sinkholes function as small sediment traps that may host valuable stratigraphic records for paleoenvironmental reconstructions and hazard assessments (e.g., subsidence, floods, hurricanes, tsunamis, tephra fall-out). The sedimentological features and completeness of such archives are influenced by the sedimentation and subsidence patterns and rates. However, karst depressions are frequently treated as static basins unaffected by settlement. This work illustrates the practicality of integrated studies combining trenching, numerical dating and shallow geophysical techniques (GPR) for characterizing the subsurface subsidence structure associated with sinkholes and reconstructing their deformational and sedimentary history. The approach is applied in two collapse sinkholes located in contrasting geomorphic settings (relict terrace and floodplain) related to deep-seated interstratal karstification of evaporites. The analysis of the sinkholes, particularly the trenching technique, provides practical information for assessing the associated subsidence hazard, including the presence of larger cavities at depth, the kinematic regime (episodic versus progressive), evidence of catastrophic displacement (fluidization structures) and the magnitude and timing of collapse events, especially the most recent one. The sinkhole located in the floodplain offers the opportunity of analyzing the possibilities and limitations of subsidence sinkholes as recorders of past floods in alluvial environments. This depression shows a largely incomplete record attributable to the high frequency of flood events compared to that of the collapse events, which create the accommodation space for sediment deposition. These limitations could be partially overcome by selecting old sinkholes situated in low terraces and/or affected by rapid subsidence.

Time domain reflectometry (TDR) potential for soil condition monitoring of geotechnical assets

The performance of geotechnical assets is influenced by various external factors including time and changing loading and environmental conditions. These changes could reduce the asset’s ability to maintain its function, potentially resulting in failure, which could be extremely disruptive and expensive to remediate; thus, the ability to monitor the long-term condition of the ground is clearly desirable as this could function as an early-warning system, permitting intervention prior to failure. This study demonstrates, for the first time, the potential of using time domain reflectometry (TDR) for long-term monitoring of the relative health of an asset (via water content and dry density) in a field trial where a clayey sandy silt was exposed to leaking water from a pipe. TDR sensors were able to provide detailed information on the variation in the soil conditions and detect abrupt changes that would relay a prompt for asset inspections or interventions. It is proposed that TDR could be used alone or together with other shallow geophysical techniques for long-term condition monitoring of critical geotechnical assets. Early-warning systems could be based on thresholds defined from the values or the relative change of the measured parameters.

Shallow geophysical techniques to investigate the groundwater table at the Great Pyramids of Giza, Egypt

Abstract. The near-surface groundwater aquifer that threatened the Great Pyramids of Giza, Egypt, was investigated using integrated geophysical surveys. A total of 10 electrical resistivity imaging, 26 shallow seismic refraction, and 19 ground-penetrating radar surveys were conducted in the Giza Plateau. Collected data for each method were evaluated by state-of-the art processing and modeling techniques. A three-layer model depicts the subsurface layers and better delineates the groundwater aquifer and water table elevation. The resistivity of the aquifer layer and seismic velocity vary between 40 and 80 Ωm and between 1500 and 2500 m s−1, respectively. The average water table elevation is about +15 m, which is safe for the Great Sphinx, but it is still subjected to potential hazards from the Nazlet El-Samman suburb where the water table elevation reaches 17 m. A shallower water table at the Valley Temple and the tomb of Queen Khentkawes, with a low topographic relief, represents severe hazards. It can be concluded that a perched groundwater table is detected in the elevated topography to the west and southwest that might be due to runoff and capillary seepage.

Is Submarine Groundwater Discharge (SGD) Important for the Historical Fish Kills and Harmful Algal Bloom Events of Mobile Bay?

Large-scale fish and crustacean kills, locally known as Jubilees, and harmful algal blooms (HABs) have been occurring in Mobile Bay (Alabama) for more than a century. In fact, the first record describing a Jubilee event in Mobile Bay during 1867 was the first ever-documented case of mass mortalities of marine animals caused by hypoxia. To evaluate the importance of submarine groundwater discharge (SGD) in the occurrence of Jubilees and HABs in Mobile Bay, a 3-year study was conducted using a multi-method approach. Significant spatial and temporal variations of SGD were revealed in the bay only by applying a combination of geochemical and shallow geophysical techniques. The development of seasonal hypoxia observed in bay waters in areas impacted by Jubilees was the result of anoxic SGD inputs, which magnitude and spatial distribution were controlled by shallow lithological heterogeneities created during the modern development of the bay. Although when compared to the river discharge SGD contributed between 0.2 (wet season) and 5% (dry season) of the total freshwater inputs to Mobile Bay, 80% of the total SGD in the bay occurred in areas ecologically impacted by hypoxia and Jubilees. In these areas, SGD comprised up to 37% of the total water inputs during the dry season, coinciding with the time of the year when Jubilees and HABs occur. In conclusion, while SGD might not be a significant source of fresh water to Mobile Bay or other estuaries worldwide, enhanced SGD caused by site-specific lithological heterogeneities can have a critical role in the development of hypoxia and ecological issues in nearshore waters.

SEISMIC MICROZONATION OF EL-FAYOUM NEW CITY, EGYPT

Seismic micro hazard zonation for urban areas is the first step towards a seismic risk analysis and mitigation strategy. Essential here is to obtain a proper understanding of the local subsurface conditions and to evaluate ground shaking effects. In the present study, an attempt has been made to evaluate the seismic hazard considering local site effects by carrying out detailed geotechnical and geophysical site characterization in El-Fayoum New City. Seismic hazard analysis and microzonation of El-Fayoum New City are addressed in three parts: in the first part, estimation of seismic hazard is done using seismotectonic and geological information. Second part deals with site characterization using geotechnical and shallow geophysical techniques. In the last part, local site effects are assessed by carrying out one-dimensional (1-D) ground response analysis using equivalent linear method by program SHAKE 2000. Finally microzonation maps have been prepared. The detailed methodology, along with experimental details, collected data, results and maps are presented in this paper.

Assessing the risk of subsidence of a sinkhole collapse using ground penetrating radar and electrical resistivity tomography

In Segovia Province, Spain, there are numerous carbonate rock outcrops that are affected by karstification processes, in which there are numerous galleries, caves and sinkholes. This is the case of the Sima de Madrona, a sinkhole collapse caused by the sinking of dolostone. The objective of this study was to ascertain the existence and the characteristics of cavities and galleries, as well as to evaluate the risk of subsidence in the area, by applying shallow and non-invasive geophysical techniques: ground penetrating radar and electrical resistivity tomography. The joint combination of these two independent shallow geophysical techniques has provided the best combination of investigation depth (up to 8m) and resolution needed to image small to medium (~1 to 4m) size cavities. The associated risks are evident because the cavities found herein affect various anthropic structures: a highway and crop fields.

Situating Remote Sensing in Anthropological Archaeology

ABSTRACTProductive applications of geophysics to anthropological questions in American archaeology necessarily involve specific research questions or agendas. While only some anthropological questions can be addressed by shallow geophysics, these techniques provide an opportunity to address some important questions that are fundamental to archaeology. One such agenda is the investigation of ‘persistent places’, which is rooted in anthropological inquiry and which can be investigated, at least in part, by shallow geophysical techniques. For the next stage in the use of remote sensing that goes beyond mere prospection, research agendas must be clearly linked with broader theoretical concepts of what we term inquiry‐based archaeogeophysics. Specifically with regard to the application of geophysics to the study of persistent places, we propose four categories of research that relate to the meaning, context and changing function of such places. These categories include: construction variation, continuity and discontinuity in the use of space, studying natural and/or cultural landscape modifications over time and space, and constancies in the use of space and architecture at the regional level. In order to illustrate these points, we provide examples from sites in Mexico and the USA that represent different time periods (2500 BC to AD 1000), adaptations (hunter–gatherers to intensive agriculturists), and levels of socio‐political complexity (egalitarian to stratified societies). The use of geophysics at these persistent places contributes to our understanding of changes in the use of space and architecture through time. Copyright © 2011 John Wiley & Sons, Ltd.

Tufas and travertines of the Mediterranean region: a testing ground for freshwater carbonate concepts and developments

AbstractActively forming travertine and tufa sites are widespread in temperate and warm climates, such as the Mediterranean region, but also develop in other climatic regimes from the Arctic to equatorial latitudes. The terms travertine and tufa are sometimes applied indiscriminately today; however, each has a specific definition. Most studied deposits are active or of Quaternary (especially Holocene) age. There are fewer Tertiary and even less late Mesozoic examples on record. In part, this is because of a recognition problem as these deposits commonly preserve as fragmentary erosional remnants. Physico‐chemical precipitation of calcium carbonate is commonly considered to be the main mechanism contributing to tufa and travertine deposition. Rapid cooling of waters further aids the process in thermal (travertine) sites. However, biomediation of calcium carbonate, associated with prokaryote–microphyte biofilms, is now recognized to be equally important in slow flowing and near static sites. Much of this precipitation occurs in close association with cyanobacterial, heterotrophic bacterial and diatoms. The Mediterranean region figures significantly in the development of travertine and tufa research. Consequently, this article employs key literature on Mediterranean examples to illustrate developments and concepts in freshwater carbonate research. Modern work started in the 1950s and defined many previously unknown outcrops. Many such sites became the targets for early radiocarbon dating studies. Since the late 1970s, a number of laboratory‐based and field‐based classification schemes have been proposed. Some schemes were based on associated macrovegetation types; others were based on petrological characteristics or on geometry and sedimentary facies characteristics. Of late, the sedimentological modelling has benefited greatly from the use of shallow geophysical techniques in order to better define the internal geometries of the deposits. Two distinct research strands were developed during the 1990s, both aimed at a better understanding of environmental change. The first considered biocomponents. The research was multifaceted and ranged from exploring the role of micro‐organisms in cement and lime mud precipitation to biostratigraphic analysis of contained pollen and invertebrate remains; these provided proxy environmental information which has helped to define past climate variability in the region. The second research strand focussed on geochemical signatures as a proxy tool for defining environmental change. This definition was made possible by new petrological studies which have greatly aided the recognition of primary depositional fabrics. This improved knowledge has reduced the sampling dangers of diagenetic contamination inherent in earlier geochemical studies. Lithofacies modelling has also brought to light important early post‐depositional neomorphic changes, which obscure primary fabrics, and complex diagenetic histories have been revealed. Stable isotope signatures of carbon and oxygen have also been used to define palaeo‐water temperature. Fine‐tuning of depositional time frames has also been achieved from absolute dating techniques employing radiocarbon and uranium/thorium.

Seismic microzonation of Bangalore, India

In the present study, an attempt has been made to evaluate the seismic hazard considering local site effects by carrying out detailed geotechnical and geophysical site characterization in Bangalore, India to develop microzonation maps. An area of 220 km2, encompassing Bangalore Mahanagara Palike (BMP) has been chosen as the study area. Seismic hazard analysis and microzonation of Bangalore are addressed in three parts: in the first part, estimation of seismic hazard is done using seismotectonic and geological information. Second part deals with site characterization using geotechnical and shallow geophysical techniques. In the last part, local site effects are assessed by carrying out one-dimensional (1-D) ground response analysis (using the program SHAKE2000) using both standard penetration test (SPT) data and shear wave velocity data from multichannel analysis of surface wave (MASW) survey. Further, field experiments using microtremor studies have also been carried out for evaluation of predominant frequency of the soil columns. The same has been assessed using 1-D ground response analysis and compared with microtremor results. Further, the Seed and Idriss simplified approach has been adopted to evaluate the soil liquefaction susceptibility and liquefaction resistance assessment. Microzonation maps have been prepared with a scale of 1:20,000. The detailed methodology, along with experimental details, collated data, results and maps are presented in this paper.

Pseudo-3D imaging of a low-slip-rate, active normal fault using shallow geophysical methods: The Geleen fault in the Belgian Maas River valley

In a detailed site survey for paleoseismic trenching, we applied shallow geophysical prospecting techniques, including ground-penetrating radar (GPR), electric resistivity tomography (ERT), and resistivity mapping to identify, locate, and visualize in 3D the Geleen fault, an active normal fault bordering the Roer Valley graben in northeast Belgium. Because of a low slip rate, the geomorphic expression of this fault is very faint in the relatively young deposits of the Maas River valley. ERT profiles show the fault as a broad, near-vertical anomaly characterized by sharp lateral resistivity contrasts, with an associated vertical offset of sediment layers. We observed offsets of [Formula: see text] and [Formula: see text] for the base and top, respectively, of a middle-late Pleistocene fluvial gravel layer. Shallow ERT and GPR profiles indicate that younger sediments are also affected by faulting, but the amount and sense of offset appear to be conflicting: ERT profiles show a near-surface, high-resistivity layer with an apparent reverse offset, and GPR profiles portray the fault as a sharp and laterally consistent disruption of reflectors, often without a clearly identifiable offset. Resistivity maps at different depths map the fault as a narrow, linear, lateral resistivity gradient matching the anomalies observed with other techniques. This method proved to be efficient in determining the precise position and orientation of dip-slip faults, and could potentially be very useful for the identification of lateral changes in fault geometry, such as splays and step-overs. Subsequent trenching confirmed the presence of a normal fault at the location predicted by the geophysical survey. Correlation with the sediments exposed on the trench walls demonstrated that, close to the surface, resistivity and dielectric permittivity contrasts mostly occur in a postdepositional soil, which developed differently on either side of the fault. This explains why shallow geophysical variations do not reflect the true fault offset.

Slope failure, mass flow and bottom current processes in the Rockall Trough, offshore Ireland, revealed by deep-tow sidescan sonar

The margins of the Rockall Trough, currently the focus of oil and gas exploration interest, incorporate an environmentallyimportant and lightly studied shelf region. To address the paucity of information within this region an extensive high resolution sidescan sonar survey was carried out in the Irish sector of the Rockall Trough in June/July 1998. This valuable natural laboratory contains information on slope stability, sediment transport, nutrient upwelling, bottom current, and biological activity at a shelf/slope/basin-floor transition. Detailed information on the nature of many of these processes can be provided from shallow geophysical techniques. The TRIM (TOBI Rockall Irish Margins) project (Shannon et al., 2001), funded by the Irish Petroleum Infrastructure Programme (Rockall Studies Group), acquired 3100 line-km of TOBI sidescan sonar and 3700 km of 3.5 kHz profiler data along the margins of the Rockall Trough (Figure 1). This project followed on from the earlier reconnaissance AIRS (Atlantic Irish Regional Survey) survey conducted over the entire southern Rockall Trough region, using the lower resolution surface towed sonar side-scan system known as GLORIA (Unnithan et al., 2001). The TOBI sidescan system, developed at the Southampton Oceanography Centre (Flewellen et al., 1993), is deep-towed at 300-400 m above the seafloor. It transmits at a frequency of 32 kHz and samples the acoustic response of the seabed and shallow sedimentary structure to a centimetre to metre depth scale. This acoustic response therefore reflects not only the backscattering properties of the seabed but also the shallow detailed structure of sediments formed by recent sedimentological processes.

Combined seismic tomographic and ultrashallow seismic reflection study of an Early Dynastic mastaba, Saqqara, Egypt

AbstractMastabas were large rectangular structures built for the funerals and burials of the earliest Pharaohs. One such mastaba was the basic building block that led to the first known stone pyramid, the >4600‐year old Step Pyramid within the Saqqara necropolis of Egypt. We have tested a number of shallow geophysical techniques for investigating in a non‐invasive manner the subsurface beneath a large Early Dynastic mastaba located close to the Step Pyramid. After discovering that near‐surface sedimentary rocks with unusually high electrical conductivities precluded the use of the ground‐penetrating radar method, a very high‐resolution seismic data set was collected along a profile that extended the 42.5 m length of the mastaba. A sledgehammer source was used every 0.2 m and the data were recorded using a 48‐channel array of single geophones spaced at 0.2 m intervals. Inversions of the direct‐ and refracted‐wave travel times provided P‐wave velocity tomograms of the shallow subsurface, whereas relatively standard processing techniques yielded a high‐fold (50–80) ultrashallow seismic reflection section. The tomographic and reflection images were jointly interpreted in terms of loose sand and friable limestone layers with low P‐wave velocities of 150–650 m s−1 overlying consolidated limestone and shale with velocities > 1500 m s−1. The sharp contact between the low‐ and high‐velocity regimes was approximately horizontal at a depth of ca. 2 m. This contact was the source of a strong seismic reflection. Above this contact, the velocity tomogram revealed moderately high velocities at the surface location of a friable limestone outcrop and two low‐velocity blocks that probably outlined sand‐filled shafts. Below the contact, three regularly spaced low velocity blocks probably represented tunnels and/or subsurface chambers. Copyright © 2005 John Wiley & Sons, Ltd.

GeoPASS: a decision support system for selecting the optimal geophysical technique for shallow surveys

In this paper we present a decision support system for selecting the optimal geophysical technique to detect shallow land targets (< 80 m-bs, i.e. less than 80 m below the surface). The system is freely available on the Internet. The aim of the decision support system is to stimulate geophysicists and end-users to assess the applicability of geophysical techniques prior to a survey in order to increase the chance of a satisfactory outcome. In the long term this will hopefully lead to enhanced recognition of the value of geophysics. The use of geophysical techniques for investigating environmental problems is less then it could be. In our view an important reason for this is that, in the Netherlands at least, geophysics in quite a number of cases does not supply the information expected by the end-user. The information is often vaguer or more ambiguous than environmental managers are used to, and/or the performance of the geophysical technique is poor due to the characteristics of the site and/or soil. The poor performance may also be caused by the application of a less suitable technique or the use of a non-optimal field-design. Applying geophysical methods without satisfactory results for end-users is, in the long term, bad for geophysics in general. Often these unsatisfactory results are due to a lack of assessment of the applicability of the geophysical techniques at specific sites prior to the survey. Such assessments hardly cost any time, especially when site characteristics are known and are backed by a geophysical decision support system. As geophysicists, we think that geophysical measurements can, if used properly, become more and more important for subsurface investigations for environmental and infrastructure projects. The well-known advantages of geophysical techniques are that a lateral continuous image of the subsurface is obtained and that geophysics is generally less destructive for the subsoil than, for example, drilling. To overcome the problem of applying non-optimal techniques, a decision support system was designed within the framework of the European HYGEIA Project (Hybrid Geophysical technology for the Evaluation of Insidious contaminated Areas). The system is called GeoPASS. In the past, several organizations developed tools for selecting a geophysical technique for a specific problem. Crosstables indicating which technique(s) could be applied to certain problems have been prepared by several organizations and companies. These tables are mostly general and do not indicate whether the technique will be successful for a specific site. A good example can be found at the website of the Swiss Geophysical Commission (www.aug.geophys.ethz.ch/gpmapping). One other geophysical decision support system asking the user more detailed information about the site characteristics is known to the authors. This system, called GAES, was specifically designed for detecting pollutants (Geophysics Advisor Expert System, Gary R. Aloft, USGS Open-File Report 92- 526). However, the system is not very user-friendly, rather old (1992), and the outcome is a set of rather general remarks about the applicability of geophysical techniques. GeoPASS is designed to provide the user with an estimate of the chance of success of several geophysical techniques. The target may be either a pollutant or a relevant subsurface feature (e.g. sealing clay layer, fracture). The outcome of GeoPASS may even be that none of the assessed geophysical techniques can detect the target given the site conditions. The aims of GeoPASS are to: ■ Provide an estimate of the success chance of geophysical techniques at a specific site and therefore avoid the application of geophysical techniques that are not appropriate ■ Make the end-user aware of the chance of success expected of the geophysical technique(s) to be applied and the factors that influence that chance ■ Give an initial estimation of the costs of a geophysical survey on the site GeoPASS also serves as a checklist. If no information about the site and target parameters is available, it is difficult to predict whether a geophysical technique is capable of supplying the required results. This situation is prone to mis-use by some enthusiastic geophysicists and geophysical companies, often resulting in disappointed end-users. Hopefully the use of GeoPASS will lead to more 'happy faces' among the end-users of shallow geophysical techniques and therefore enhance recognition of the value of geophysical techniques in general.

The recognition of barrage and paludal tufa systems by GPR: case studies in the geometry and correlation of Quaternary freshwater carbonates

Abstract Tufas provide virtually the only sedimentary and proxy-environmental records within karstic terrains. However, they are difficult to access. Shallow geophysical prospecting techniques, such as resistivity and shallow seismic reflection, fail to define the often complex internal bedform details in tufa deposits and many deposits appear too well lithified to auger-sample. Nevertheless, the application of ground penetrating radar (GPR) permits the recognition of up to five distinct types of radar reflectors that can be directly related to distinct lithologies commonly seen in tufa cores: (1) well-lithified phytoherms produce sharp, sinuous and often complexly truncated bright signals; (2) soft lime muds produce subhorizontal, laterally continuous lower contrast (dull) laminar bedform signals; (3) organic-rich deposits (sapropels and peats) produce poorly focused dull responses, often with internal ‘noise’; (4) the tops of bladed and coarse-grained deposits, such as flint gravel, give a strong bright signal; and (5) the associated presence of clay-grade lime silts and muds within the top of gravel beds produces the same top-bed signal as 4, but internal details of the deposit are masked and a remarkably homogeneous dull signal response is typical throughout the lower parts of the deposit. From these GPR responses it is possible to make meaningful three-dimensional comparisons of the internal geometries of Holocene tufa deposits. Problematic tufa deposits in the valleys of the Derbyshire Wye and the Hampshire Test, UK, are presented to illustrate the universal value of GPR surveying for fresh-water carbonate recognition and for providing key information on valley-bottom resurgence locations.

Electrical resistivity ground imaging (ERGI): a new tool for mapping the lithology and geometry of channel‐belts and valley‐fills

AbstractEfforts to map the lithology and geometry of sand and gravel channel‐belts and valley‐fills are limited by an inability to easily obtain information about the shallow subsurface. Until recently, boreholes were the only method available to obtain this information; however, borehole programmes are costly, time consuming and always leave in doubt the stratigraphic connection between and beyond the boreholes. Although standard shallow geophysical techniques such as ground‐penetrating radar (GPR) and shallow seismic can rapidly obtain subsurface data with high horizontal resolution, they only function well under select conditions. Electrical resistivity ground imaging (ERGI) is a recently developed shallow geophysical technique that rapidly produces high‐resolution profiles of the shallow subsurface under most field conditions. ERGI uses measurements of the ground's resistance to an electrical current to develop a two‐dimensional model of the shallow subsurface (<200 m) called an ERGI profile. ERGI measurements work equally well in resistive sediments (‘clean’ sand and gravel) and in conductive sediments (silt and clay). This paper tests the effectiveness of ERGI in mapping the lithology and geometry of buried fluvial deposits. ERGI surveys are presented from two channel‐fills and two valley‐fills. ERGI profiles are compared with lithostratigraphic profiles from borehole logs, sediment cores, wireline logs or GPR. Depth, width and lithology of sand and gravel channel‐fills and adjacent sediments can be accurately detected and delineated from the ERGI profiles, even when buried beneath 1–20 m of silt/clay.

The use of geophysical prospecting for imaging active faults in the Roer Graben, Belgium

As part of a paleoseismological investigation along the Bree fault scarp (western border of the Roer Graben), various geophysical methods [electrical profiling, electromagnetic (EM) profiling, refraction seismic tests, electrical tomography, ground‐penetrating radar (GPR), and high‐resolution reflection seismic profiles] were used to locate and image an active fault zone in a depth range between a few decimeters to a few tens of meters. These geophysical investigations, in parallel with geomorphological and geological analyses, helped in the decision to locate trench excavations exposing the fault surfaces. The results could then be checked with the observations in four trenches excavated across the scarp. Geophysical methods pointed out anomalies at all sites of the fault position. The contrast of physical properties (electrical resistivity and permittivity, seismic velocity) observed between the two fault blocks is a result of a differences in the lithology of the juxtaposed soil layers and of a change in the water table depth across the fault. Extremely fast techniques like electrical and EM profiling or seismic refraction profiles localized the fault position within an accuracy of a few meters. In a second step, more detailed methods (electrical tomography and GPR) more precisely imaged the fault zone and revealed some structures that were observed in the trenches. Finally, one high‐resolution reflection seismic profile imaged the displacement of the fault at depths as large as 120 m and filled the gap between classical seismic reflection profiles and the shallow geophysical techniques. Like all geophysical surveys, the quality of the data is strongly dependent on the geologic environment and on the contrast of the physical properties between the juxtaposed formations. The combined use of various geophysical techniques is thus recommended for fault mapping, particularly for a preliminary investigation when the geological context is poorly defined.

Geoelectric resistance scanning on parts of Abydos Cemetery Region, Sohag Governorate, Upper Egypt

The ancient Egyptian Abedju (original name of the present Abydos) locality is a famous archaeological site southwest of Balyana town, Sohag Governorate, Upper Egypt. It is located to the west of the agricultural land of the Nile Valley. The locality represents one of the most important burial grounds for kings and high court dignitaries in ancient Egypt. Shallow geophysical techniques are considered as one of the most accurate and cost-effective methods used in archaeological prospecting and are considered as rapid and safe techniques in detecting a wide range of buried archaeological features. The geoelectric resistance scanning technique, using the Geoscan RM-15 Resistance Meter, is applied with a twin-electrode con- figuration at three sites of the unexcavated localities of Abydos area. This technique proved to be a useful means of exploring the sites through mapping the subsurface burial inhomogeneities resulting from the marked resistance contrast between the buried stone and brick walls and the covering fill of sand, silt and clay. Interesting different buried features are imaged and displayed on maps and three-dimensional representations to guide the archaeological excavation programmes at the sites of Umm El-Qaab, Shunet El-Zebib and Kom El-Sultan within the Abydos region. At the Umm El-Qaab site the combined interpretation of the tomography resistance ranges and maps indicate buried walls and rubble as well as an amorphous background area. At the Shunet El-Zebib site, the texture of the images, which show scattered and crowded high-resistance anomalies, confirms the exist- ence of buried tombs at the site, which is surrounded by defence walls. Thus, this site was not a fortress or a palace, but a special cemetery for higher dignitaries. Kom El-Sultan, however, shows a concentration of high-resistance anomalies at the northern part. Thus, these may be related to walls and rubble related to a nearby elevated temple. Copyright * c 1999 John Wiley & Sons, Ltd.

Shallow geophysical investigations on Tell el Farama, Northwest Sinai, Egypt

The main objective of the present study was to locate the buried archaeological remains in the area of Tell el Farama in northwestern Sinai, using shallow geophysical techniques. Magnetic and electric resistivity surveys were carried out on a selected area of about 300 m×330 m, which represents the present location of the ancient Citadel of el Farama. The field data have been collected, automatically analysed and presented in the form of maps and profiles, which show several magnetic and resistivity anomalies of different amplitudes and shapes. After the interpretation of these geophysical anomalies, a broader distribution of the buried archaeological features has been estimated and a simplified interpretative map is given to assist future plans for excavation of this area. This interpretation was based mainly on the magnetic results. The geoelectric resistivity method failed to give an indication of the distribution of the archaeological remains in this area due to the high salinization, shallow water table and soil moisture regime in the area of el Farama. Copyright © 1998 John Wiley & Sons, Ltd.

Engineering aspects of the chalky boulder clay at the new town of Milton Keynes in Buckinghamshire

Summary The paper outlines some engineering aspects of tills, particularly at Milton Keynes, and demonstrates methods for their investigation. The methods discussed include the display in vertical section of geotechnical index parameters determined for samples taken from a trench, shallow geophysical techniques and large scale in situ testing. These are all seen as complementary to a preliminary geological investigation. The objects of this specific study were twofold: to establish the variability of engineering properties with its consequent effect on representability of results and necessary sampling interval, and to investigate the presence of depositional features detrimental to engineering operations in tills, particularly at the Milton Keynes site. The variability was found to be so great that borehole methods were not applicable. A pre-existing shear plane extending from ground surface to about 3.5 m depth was also discovered. These considerations prompted the recommendation of special investigation procedures to suit the circumstances.